Cat acting weird

My kitten or cat, shes about 6 months old is suddenly acting weird. Shes crouching and walking very low to the floor and shaking her tail almost involuntarily. I'm a bit worried because I dont know if this could be bad or if she is hurt. What does this mean?

Answer from KateHiIs she spayed? If not then her behavior could her going into heat. Please see this page for more on this

https://www.our-happy-cat.com/cat-in-heat.html

If she is spayed then it would be a good idea to take her to the vets for a check up as she may have some discomfort in her rear end.